Series ignition systems in contemporary internal combustion engines which are embodied as spark ignition engines having been operating for many decades according to the simple and reliable principle of coil discharge, i.e. an ignition coil which is configured as a transformer is charged partially as far as its saturation range on the primary side in accordance with its inductance from the vehicle on-board power system voltage. At the ignition time, the charge is interrupted by means of an electronic switching operation, for example by an ignition-IGBT (Insulated Gate Bipolar Transistor). As a result, a voltage of, for example, 5 kV to 35 kV is built upon the secondary side and gives rise to a flashover in the spark gap of the sparkplug in the combustion chamber of the internal combustion engine. The energy which is stored in the coil is subsequently dissipated in the ignition plasma.
In the course of the progressive development of engines, it has been necessary to implement reductions in terms of consumption and emissions, and in the last few years these have consequently placed an increasing additional burden on the ignition system and will continue to do so in the future. Examples of this are, for example, stratified combustion in which liquid fuel components with high flow rates impede the spark discharge and bring about numerous new spark formations. Rising combustion chamber pressures for improving the engine efficiency also increase the breakdown resistance in the spark gap and bring about an increase in the breakdown voltage which also influences the sparkplug wear. In future highly charged engine generations the latter will give rise to secondary-side voltage increases far beyond 35 kV. Both the rising breakdown voltages and the flow states which become more intensive at the sparkplug have a tendency to shorten the spark duration of the sparks since ever larger proportions of the energy stored in the coil have to be made available to build up and maintain the spark. A much more promising trend in the development of new combustion methods is the use of multiple sparks, in which the coil energy is transmitted efficiently to the mixture at short intervals, which increases the inflammation reliability.
In application DE 10 2009 057 925.7, which was not published before the priority data of the present document, an innovative method for operating an ignition device for an internal combustion engine and an innovative ignition device for an internal combustion engine for carrying out the method are described. Accordingly, an ignition device for an internal combustion engine is formed with an ignition coil which is embodied as a transformer, a sparkplug which is connected to the secondary winding of the ignition coil, a controllable switching element which is connected in series to the primary winding of the ignition coil, and a control unit which is connected to the primary winding of the ignition coil and to the control input of the switching element. The control unit makes available an adjustable supply voltage for the ignition coil and a control signal for the switching element as a function of the currents through the primary winding and the secondary winding of the ignition coil and as a function of the voltage between the connecting point of the primary winding of the ignition coil to the switching element and to the negative terminal of the supply voltage. The method for operating this device has the following sequence in this context:
in a first phase (charging), the switching element is switched on at a first switch-on time through the control signal and switched off again at the predefined ignition time,
in a subsequent second phase (breakdown), the primary voltage or a voltage derived therefrom is compared with a first threshold value, and when this voltage undershoots the first threshold value the switching element is switched on again at a second switch-on time,in a subsequent third phase (arc) the supply voltage is regulated in such a way that the current through the secondary winding of the ignition coil corresponds approximately to a predefined current, and the current through the primary winding of the ignition coil is compared with a predefined second threshold value, and when this current exceeds the second threshold value the switching element is switched off again at a first switch-off time,in a subsequent fourth phase (breakdown), the current through the secondary winding of the ignition coil is compared with a third threshold value, and when this current undershoots the third threshold value the switching element is switched on again at a third switch-on time,the third and the fourth phases are, if appropriate, subsequently repeated until a predefined spark duration is reached under the time at which the switching element is definitively switched off.
A corresponding device is illustrated in FIG. 1, and the time profile of the significant voltages and currents is illustrated in FIG. 2.
Investigations into the basic principles of internal combustion engines have shown that the interaction of the internal cylinder flow with a spark of the sparkplug has a considerable influence on the spark itself as well as consequently on the quality of the ignition and inflammation of various mixture states. Even in the case of weak flow states far below 5 m/s, the spark is deflected in the spark gap, which deflection becomes continuously larger as the effect persists.
Moderate flow speeds have a positive effect on the running of the engine despite shortening of the spark duration since they have a tendency to increase the spark volume and improve the transmission of heat to the surrounding mixture. In terms of ignition technology, particularly in the short range area, in a few millimeters around the sparkplug prove problematic since both the ignition hook and the sparkplug body itself constitute considerable heat sinks and large portions of the heat in the plasma are absorbed in the form of radiation, convection or simply thermal conduction and are unavailable for heating the mixture. Even after successful ignition, these heat sinks impede the initial growth of the flame and delay the combustion sequence which is so critical at the beginning.
The introduction of multiple sparks or series sparks improves the situation by virtue of the fact that an intermittent supply of energy with simultaneous extension over time slightly increases the ignition probability when there is a lack of homogenization of the mixture. Although through the extension over time the ignition times become slightly imprecise, on the other hand greater extension of the plasma is promoted given a sufficiently high spark frequency and spark energy content. Even relatively high spark energies can improve the inflammability at the cost of increased sparkplug wear.
The maximum extension of the plasma which can be achieved as a function of the combustion chamber pressure and flow (turbulence) and which is specific to the respective operating state of the engine and defines the most efficient “far-range area” of the sparkplug in terms of inflammation technology has not been considered until now.